The present invention relates generally to a steering apparatus for motor vehicles, and more particularly to a steering control apparatus for properly controlling the steering angle of the front or rear wheels in accordance with the travelling state of a motor vehicle.
Various types of steering apparatuses for motor vehicles have been developed heretofore, one known approach being to control a servo valve by a hydraulic pressure mechanism and another known approach being to use a dc servo motor or a stepping motor. These steering apparatuses are adapted to drive the servo valve or the servo motor by a current corresponding to the difference between the aarget steering angle and the actual steering angle under the so-called PID (Proportional Integral Differential) control or state feedback control. While the steering systems require increasing the amount of the current, i.e., control gain, in order to improve responsibility and control accuracy, the control gain increase results in overshoot with respect to the target steering angle and hunting, i.e., periodic variations, occurring in the vicinity of the target steering angle in response to variations in the power source voltage and fluctuations in load torque during the steering operation, thereby making unstable the driving characteristics of the motor vehicle. On the other hand, prevention of the overshoot and hunting leads to lowering the responsibility and control accuracy which in turn makes worse the driving characteristics.
In order to avoid such problems, it is known that the PID control steering system may have an arrangement for feedback control of various signals, the state feedback control system having an arrangement for successively changing the control gain in accordance with the real time system identification. However, provisions of these arrangements make complex the control circuits, resulting in complex steering systems.